1. Field of the Invention
The present invention relates to a fluid-filled vibroisolating device having an expandable and contractable fluid chamber made of an elastomeric material or the like and filled with a fluid, and more particularly to a fluid-filled vibroisolating device for use as a vibroisolating or vibration damping support for a power unit such as an automotive engine or the like.
2. Description of the Relavent Art
Motor vehicles such as automobiles develop various vibrations of different frequencies and amplitudes dependent on operating conditions such as engine rotational speeds, varying road surfaces, and the like. Therefore, the motor vehicles are required to be equipped with vibroisolating or vibration damping devices capable of absorbing or damping vibrations in a wide range of frequencies and amplitudes.
Known vibroisolating devices include a fluid-filled vibroisolating device comprising a support member on which a vibrating body such as an engine is mounted, a support member installed on a support body such as a vehicle frame, and an elastomeric body of rubber, for example, having opposite ends fixed to the support members and defining a main fluid chamber therein. The fluid-filled vibroisolating device also includes an auxiliary fluid chamber communicating with the main fluid chamber through an orifice. The main and auxiliary fluid chambers are filled with a noncompressible fluid such as water, oil, or the like.
The main and auxiliary fluid chambers are divided by a partition which has the orifice providing fluid communication between the main and auxiliary fluid chambers. Vibration of the engine is generally absorbed and/or dampened by elastic deformation of the elastomeric body and the flow of the fluid through the orifice between the main and auxiliary fluid chambers.
There has been a demand for improved dynamic spring characteristics and improved damping capability for such fluid-filled vibroisolating devices. Japanese Laid-Open Patent Publication No. 60-263736 discloses a fluid-filled mount as one example of the fluid-filled vibroisolating device. The publication discloses that by causing the fluid flowing through the orifice to resonate, a loss coefficient, i.e., a damping coefficient at a desired frequency of vibration can be increased and a dynamic spring coefficient can be reduced.
Though the disclosed fluid-filled mount somewhat improves its dynamic spring characteristics and damping characteristics, it is desired that these characteristics should be more improved.
More specifically, the elastomeric body of the mount which substantially defines the main fluid chamber is low in rigidity in a direction (hereinafter referred to as an expanding direction) normal to the direction of vibration, and tends to collapse when the exciting force for developing the vibration is large. To eliminate this drawback, there has been proposed in recent years a mount including an intermediate reinforcing member which reinforces an elastomeric body to increase its rigidity. In the proposed mount, the elastomeric body which defines a main fluid chamber is divided into two elastomeric members which are joined to each other by the intermediate reinforcing member for thereby increasing the rigidity of the entire elastomeric body in the expanding direction to guard against collapsing under strong vibration. Since, however, the intermediate reinforcing member which has a relatively large mass is positioned between the vibrating elastomeric members, these members constitute a vibratory system causing the intermediate reinforcing member to vibrate. As shown in FIG. 8, the vibration of the intermediate reinforcing member is liable to give rise to peaks of the dynamic spring constant of the entire system at medium and high frequencies, with the result that the transmission of vibration in certain frequency ranges, particularly medium and high frequency ranges, to the vehicle frame cannot effectively be reduced. To minimize adverse effects resulting from the use of the intermediate reinforcing member, the shape and weight of the intermediate reinforcing member have to be strictly designed, and the range of shapes and weights thereof that are available is considerably limited.
The orifice may be formed in various shapes in order to cause the fluid flowing therethrough to flow desirably or resonate at approropriate timing. An orifice shape has been desired which can achieve a uniform reduction in the dynamic spring constant in a wide range of frequencies, particularly a high frequency range.
In view of the problems of the conventional fluid-filled vibroisolating device, it is an object of the present invention to provide a fluid-filled vibroisolating device which has an improved damping capability and can reduce the dynanic spring constant uniformly in a wide range of frequencies.
According to the present invention, there is provided a fluid-filled vibroisolating device comprising a joint member adapted to be joined to a vibrating body such as an engine, a support member adapted to be supported on a supporting body such as a vehicle frame and defining an expandable and contractable auxiliary fluid chamber filled with a fluid, an elastomeric member interconnecting the joint and support members and disposed in a vibrating direction in which the vibrating body vibrates, the elastomeric member, the joint member, and the support member jointly defining an expandable and contractable main fluid chamber filled with a fluid, a partition mounted in the support member and separating the main and auxiliary fluid chambers from each other, the partition having flow regulating means for regulating the flow of the fluid between the main and auxiliary fluid chambers, and a reinforcing member integrally formed with the elastomeric member for preventing the elastomeric member from being collapsed. Various parameters of the fluid-filled vibroisolating device are selected to approximately meet the equation: ##EQU1## where SE is the effective fluid draining area which contributes to a change in the volume of the main fluid chamber when the joint member is displaced with the support member fixed, Si is the effective fluid draining area which contributes to a change in the volume of the main fluid chamber when the reinforcing member is displaced in the vibrating direction with the joint and support members fixed, kl is the static spring constant when the joint member is displaced in the vibrating direction with the main fluid chamber open and the reinforcing member fixed, k is the static spring constant when the joint and support members are relatively displaced in the vibrating direction with the main fluid chamber open, and K is the static spring constant when the joint and support members are relatively displaced in the vibrating direction with the flow regulating means closed.
With this construction, the reinforcing member is prevented from being vibrated, and the dynamic spring characteristics of the entire device is not affected by the reinforcing member. The dynamic spring constant of the device is uniformly lowered in a full range of vibration frequencies for thereby reducing vibration transmitted.
The partition has a storage chamber defined therein and held in communication with the main and auxiliary fluid chambers through a plurality of first orifices opening into the main fluid chamber and a plurality of second orifices opening into the auxiliary fluid chamber in coaxial relation to the first orifices as pairs. The flow regulating means comprises a movable plate movably disposed in the storage chamber and movable at least axially of the first and second orifices dependent on the difference between fluid pressures in the main and auxiliary fluid chambers, the first and second orifices having openings opening toward the movable plate, the openings of at least one of the first and second orifices having tapered portions flaring toward the movable plate.
When vibration of a small amplitude is applied to the device, the movable plate is moved to absorb a change in the fluid pressure, and the elastomeric member is prevented from resonating through resonant action due to the mass of the fluid in the orifices and a spring component of the elastomeric member relative to the fluid pressure, so that the dynamic spring characteristics can be prevented from being lowered. The movable plate in the storage chamber has a small area of contact with other members, for effectively constributing to the absorption of changes in the fluid pressure and the prevention of resonance of the elastomeric member. When vibration with a large amplitude is applied, the movable plate is not moved, but the fluid pressure is increased for higher vibration damping capability.
The partition comprises an upper plate comprising a base plate in the form of a thin metallic sheet and an elastomeric body bonded thereto, and a lower plate in the form of a thin metallic sheet. Therefore, orifices of complex shape can be defined in the partition, and any burrs on the partition can easily be removed.
The above and further objects, details and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, when read in conjunction with the accompanying drawings.